System for detecting gaseous contaminants in air

ABSTRACT

A system for detecting the presence in air of selected gaseous contaminants. The system includes an aerosol detector arranged to monitor the air including a first electrode and a second perforate electrode defining a chamber within which said first electrode is disposed and which is open to ambient atmosphere; a radioactive source for dissociating air molecules within said chamber; and a vapor emitter for introducing to the air a vapor selected to form an aerosol by chemical reaction with said gaseous contaminants.

limited States Patent [191 Gentry Dec. 16, 1975 SYSTEM FOR DETECTINGGASEOUS CONTAMINANTS llN AIR [75] Inventor: William O. Gentry, OakRidge,

Tenn.

[73] Assignee: The United States of America as represented by the UnitedStates Energy Research and Development Administration, Washington, DC.

22 Filed: on. 30, 1973 21 Appl. No.: 411,128

[52] US. Cl 23/254 E; 23/232 E; 23/255 E; 73/23; 340/237 R [51] Int. CL?G01N 27/62; G01N 33/00; 6088 21/00 [58] Field of Search 23/254 E, 254 R,255 E, 23/255 R, 232 E; 340/237 R; 73/339 R, 23, 421.5; 21/109 [56]References Cited UNITED STATES PATENTS 2,702,471 2/1955 Vonnegut 340/2372,702,898 2/1955 Meili et a1.... 340/237 Vonnegut 23/255 R 3,163,49412/1964 Kaye 21/109 3,198,721 8/1965 Rich 23/255 R 3,427,880 2/1969Grobel et a1 73/339 3,522,431 8/1970 Coffey et a1. .v 23/254 R 3,674,4357/1972 Van Luik, Jr. et a1 23/254 R 3,693,009 9/1972 Sasaki 250/3893,751,969 8/1973 Scbrock .1 23/254 E Primary Examiner-Barry S. RichmanAssistant Examiner-Dale Lovercheck Attorney, Agent, or FirmJohn A.Horan; David S. Zachry; Fred 0. Lewis [5 7] ABSTRACT A system fordetecting the presence in air of selected gaseous contaminants. Thesystem includes an aerosol detector arranged to monitor the airincluding a first electrode and a second perforate electrode defining achamber within which said first electrode is disposed and which is opento ambient atmosphere; a radioactive source for dissociating airmolecules within said chamber; and a vapor emitter for introducing tothe air a vapor selected to form an aerosol by chemical reaction withsaid gaseous contaminants.

10 Claims, 3 Drawing Figures US. Patent Dec. 16, 1975 Sheet 1 of 2nlnllnlllnll,

m \f/ l IIIIIIIIIIIIIIIIIA US. Patent Dec. 16, 1975 Sheet 2 of23,926,560

mA/// W n w I SYSTEM FOR DETECTING GASEOUS CONTAMINANTS IN AIRBACKGROUND OF THE INVENTION This invention was made in the course of, orunder, a contract with the United States Atomic Energy Commission.

This invention relates generally to apparatus for the detection of aselected component of a gas mixture and more particularly to a systemfor the detection of selected gaseous contaminants in air, said systemincluding an aerosol detector.

It is well known that ionization chambers or condensation nucleidetectors can be used to monitor air to give warning of the presence ofparticulates resulting from thermal decomposition or from combustion.For example, US. Pat. No. 2,702,898 to Meili et al describes in detail asmoke-detection system wherein an electrostatic field is establishedbetween the electrodes of an ionization chamber which is exposed to air.A radioactive source is provided to ionize the air in the chamber, andmeans are provided to give warning of the reduction in ion current flowwhich results from the presence of visible or invisible particles (e.g.,smoke) in the air. Again, US. Pat. No. 3,427,880 to Grobel et a]describes in detail the use of either an ionization chamber particledetector or a condensation nuclei detector to monitor gas which has beencirculated through electrical machinery to cool the same. Overheating ofthe machinery decomposes a material coated thereon, generating submicronparticles which are entrained in the cooling gas. If an ionizationchamber is used for monitoring the cooling gas, the presence of thesubmicron particles is indicated by a decrease in ion current flow. If acondensation nuclei detector is employed, a sample of the cooling gas ishumidified and then expanded, causing the water vapor to condense on anysubmicron particles present. The condensation in turn changes the amountof scattered light incident on a photo tube; as a result the output fromthe tube changes, indicating the presence of the submicron particles.Such methods of detection are effective because they are sensitive tothe presence of the visible and invisible particles generated bydecomposition or combustion. Unfortunately, such methods are unsuitablefor the detection of gaseous contaminants not containingparticulates-for exam ple, gases such as hydrogen fluoride (HF),fluorine (F sulfur dioxide (S and nitrogen dioxide (N0 SUMMARY OF THEINVENTION Accordingly, it is an object of this invention to provide anovel system for the detection of selected gaseous contaminants in air.

It is another object to provide a system wherein an aerosoldetectore.g., an ionization chamber particle detector or a condensationnuclei detector-can be used to detect the presence of certainnonparticulate gaseous contaminants.

It is another object to provide a system for giving warning of thepresence in air of not only decomposition and combustion products butalso selected gaseous contaminants substantially free of particulates.

This invention can be summarized as follows:

A system for indicating the presence of a selected gaseous contaminantin air comprising the combination of an aerosol detector for monitoringsaid air and a vapor emitter introducing to the air so monitored a vaporselected to form an aerosol by chemical reaction with said gaseouscontaminant.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of aconventional ionization chamber smoke detector as modified in accordancewith this invention;

FIG. 2 is a schematic diagram of an alternative form of the vaporemitter 2 shown in FIG. 1; and

FIG. 3 is a detail view (not to scale) of a wick-and evaporator assemblyshown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention is applicable tothe detection of various gaseous contaminants in wet or dry air. Forbrevity, however, it will be illustrated chiefly as applied to thedetection of corrosive fluorine'containing gases in the atmosphere.

FIG. 1 illustrates one form of this invention as designed for thedetection of HF in the atmosphere. The system includes a conventionalaerosol detector 1 and a specially provided source 2 of a gaseousreagent, to be described hereinafter. The term aerosol detector is usedherein to refer to means for indicating or otherwise giving warning ofthe presence of visible and/or invisible particles in the gas mixture ofintereste.g.,

air.

In the form shown in FIG. 1, the new detection system includes a pair ofionization chambers 3 and 10 and a circuit for operating the chambersand giving warning of the presence of HF in the atmosphere. The chamber3, which serves as a sensing chamber, includes a central electrode 5 anda cylindrical outer electrode 7. The electrode 7 is formed with a metalscreen portion, so that the interior of the sensing chamber is open tothe atmosphere. Mounted within chamber 3 are sources 9 and 11 ofionizing radiation-e.g., alpha sourcesfor dissociating air molecules inthe chamber into electrons and positive ions. The central electrode 5extends through a port in the wall of cylinder 7, and the outer end ofthis electrode is connected to an impervious metal shell 8 enclosing areference ionization chamber 10. The metal shell 8 and a central supportpost 13 combine to form the negative electrode of the reference chamber.The positive electrode is a circular plate 12 having an aperture at itscenter. The electrode 12 is insulated from the shell 81 and ispositioned in the region between the portions 8, 13 of the negativeelectrode. The shell 8 enclosing chamber 10 is charged withparticle-free air and is virtually isolated from the externalatmosphere. This chamber contains alpha-radiation sources 15 and 17 fordissociating air molecules.

As shown, the outer electrode 7 of the sensing chamber 3 is connected tothe negative side of a suitable dc. power supply 19, whereas the plateelectrode 12 of the reference chamber 10 is connected, through theactuating coil of a conventional electromagnetic relay 21, to thepositive side of the supply. Thus, a potential is applied across theserially connected chambers 3 and 10. As a result, electrons andpositive ions produced by the above-mentioned dissociation are attractedto the chamber electrodes, producing a very small current flow throughthe chambers. As is well known, the introduction of microscopic orsubmicroscopic particles into the sensing chamber 3 will produce arelatively abrupt decrease in the ionic flow therein-i.e., will producean abrupt increase in the resistance of this chamber. The referencechamber is designed to operate under saturated conditions and serves asa constant-current device with very high dynamic resistance. Thus, arelatively small increase in the resistance of chamber 3 due toparticles in the atmosphere therein will be reflected in a relativelylarge change in the voltage at the junction (shell 8) of the chambers.

A commercially available gas-discharge triode 29, designed for operationwith high-impedance ion chambers, has its anode connected to thereference chamber electrode 12 and its cathode connected to the cylinder7. A starter electrode 31 of the tube is connected to a potential pointcommon to the chambers, so that the potential maintained on the starterelectrode 31 is dependent on the ion current through the chambers. Ifsufficient particles are introduced to the sensing chamber 3, thevoltage between the starter electrode and the cathode increases to aselected value and a discharge takes place therebetween. This in turntriggers relatively heavy current flow between the cathode and theanode, actuating relay 21. Actuation of the relay closes a contact 33connected in series with an electrical alarm 35 and a power supply 37,thus energizing the alarm.

In accordance with this invention as applied to detecting I-IF, a vaporemitter 2 is provided for continuously introducing a selected vapor intothe sensing ionization chamber 3. The vapor so introduced is one whichin the gaseous phase reacts with HF to form particles, thus forming anaerosol. The vapor emitter may, for example, be a molecular sievematerial whose pores have been saturated with dibutylamine. In thearrangement shown, the saturated molecular sieve material is containedin an open-topped canister 39 which is mounted to the underside of thescreen portion of the sensing-chamber electrode 7. Thus, dibutylaminevapor continuously flows from the canister into the sensing chamber. IfHF is present in the atmosphere monitored by chamber 3, vapor from thecanister reacts therewith to form an aerosol. Assuming a thresholdconcentration of HF, the resulting increase in the resistance of thesensing chamber energizes the alarm 35 as described above.

EXAMPLE I A system of the kind shown in FIG. 1 was built and tested. Thechambers 3, 10 and the tube 29 constituted a Model F-SB Pyr-A-Larm,manufactured by Pyrotronics, Inc., Cedar Knolls, NJ. The normaloperating voltage applied across the series combination of the chamberswas 220 volts do. The vapor emitter 2 was a canister (length, 2% inches;diameter, /4 inch) filled with a dibutylamine-saturated molecular sievematerial (such as Type 3A Molecular Sieve, manufactured by Union CarbideCorporation, Linde Division, 270 Park Ave nue, New York, N.Y.). Themolecular sieve had been saturated by soaking it in liquid dibutylaminefor 24 hours.

The system was mounted in a test chamber and tested by introducingvarious gaseous mixtures of HF and air to the chamber. Table Isummarizes the results obtained.

TABLE l-continued HF in Air Response Time Run No. (ppm) (sec.)*

Time in seconds between introduction ot the sample to the test chnmherand the sounding ofthc alarm The sample was introduced at a point 6"from the detector.

Tests indicated that the vapor emitter generated vapor satisfactorilyfor detection purposes for approximately 30 days.

EXAMPLE II The arrangement shown in FIG. 1 was modified by substitutinga different vapor source. As shown in FIGS. 2 and 3, this sourceincluded a stainless steel reservoir 41 having a top outlet tube 43. Theexternally threaded tip of the outlet tube was inserted in an aperturein the base of the shell 7 of sensing chamber 3 and clamped thereto bymeans of nuts 45, 46. As shown in FIG. 3, the inner wall of the uppernut 45 was grooved for reception of an annular Teflon washer for sealingagainst the upper end of the tube 43. Fitted in the aperture of thewasher was a porous glass rod, or evaporator, 51 which was positioned toextend within the sensing chamber and terminate just below the centralelectrode 5. The upper end of the wick 49 was wrapped around and securedto the base of the evaporator. The reservoir was filled with grams ofliquid dibutylamine, the liquid being introduced by means of ahypodermic needle inserted through a fine hole 53 in the tube 43. Thewick conveyed liquid dibutylamine from the reservoir to the evaporator,from which the dibutylamine evaporated into the sensing chamber at arate which could be varied by adjusting the amount of evaporator surfaceexposed to the chamber. The detection system operated satisfactorily asan HP detector for over 5 months at atmospheric temperature and pressureuntil the supply of dibutylamine essentially was exhausted. The averageevaporation rate for the dibutylamine was about 0.4 gram/day. Thefollowing table illustrates the results obtained.

Additional tests established that the system utilizing a vapor emitterof the type illustrated in FIGS. 2 and 3 would give warning of 2-10 ppmN0 or S0 in air within 30 seconds.

The systems described above are merely illustrations of this invention.If desired, only one ionization chamber need be employed, as describedin U.S. Pat. No. 3,693,009, to Sasaki. The particles can be sensed bymeans of a condensation nuclei detector of the kind described inabove-referenced U.S. Pat. No. 3,427,8 80, or they can be sensed withany other suitable aerosol detector, such as a conventional gas streammonitor utilizing beta radiation. Ionization chambers are preferred,however, because of their convenience, their ability to operatecontinuously, and their high sensitivity, and because they do notrequire moving parts.

It is within the scope of this invention to detect the presence in theatmosphere of any gaseous contaminant selected from the group consistingof corrosive, fluorine-containing gases (for example: HF, BrF and F andgaseous pollutants such as S0 N0 and HCl. The vapor used for detectioncan be supplied by any volatile material which in the gas phase reactswith the corrosive gas or gaseous pollutant to form an aerosol. That is,the vapor used for detection can be selected on the basis of variousreactions known in the art. For example, any amine derived from ammoniaby the replacement of hydrogen with one or more univalent hydrocarbonradicals can be used in accordance with this invention for the detectionof S0 N0 HCl, and corrosive fluorine-containing gases. Primary,secondary, and tertiary amines are especially suitable.

It will be apparent that detection systems designed in accordance withthis invention have numerous applications. For instance, systems of thekind shown in FIGS. l-3 can be used to advantage in plants for theproduction of UF from U0 or U0 Such plants typically employ both gaseousHF and gaseous F to effect the conversion to UF Thus, the systems shownin the figures can be used to monitor the building atmospherecontinuously for F and HF, as well as gaseous products from thermaldecomposition or combustion.

This invention has been illustrated herein as applied to the detectionof gaseous contaminants such as S0 N0 HCl, and corrosivefluorine-containing gases. As indicated, the selection of a suitablevapor for forming an aerosol with such contaminants is within the skillof the art. In applications where the atmosphere is being monitored onlyfor corrosive fluorine-containing gases, the vapor can, for example, beammonium hydroxide, dimethylamine, or dibutylamine. Where S0 and N0 arethe pollutants of interest, the vapor can, for example, be methylamine,dimethylamine, or dibutylamine. Vapors useful in the detection of HClinclude dibutylamine. ammonium hydroxide, and ammonia. Other suitablevapors will be apparent to those versed in gasphase reactions yieldingsolid products.

The vapor employed in accordance with this invention can be derived inany suitable manner from any suitable source. For example, the vapor canbe generated by evaporation from a body of liquid, sublimation from thesolid state, or volatilization of a material absorbed in or adsorbed onanother material. If desired, the source may be heated to promotevaporization. Again, the vapor can be derived from a gas supply, as froma cylinder of gas under pressure. (The term vapor is used herein torefer to a substance in the gaseous state but below its criticaltemperature.) In those instances where the vapor is derived byvolatilizing a sorbed material, preferably the material is sorbed on ahigh-surface-area substance, such as a molecular sieve or activatedcharcoal or alumina. Dibutylaminecontaining reservoirs of the kindillustrated in FIG. 2 but not containing a wick assembly have been foundto provide a sensitivity in the range of -100 ppm HF in air.

The vapor emitter 2 can be provided at any convenient location ensuringthat vapor therefrom is introduced to the atmosphere being monitored bythe aerosol detector. The emitter may be mounted to the aerosoldetector, as described. Alternatively, it may be mounted in the vicinitythereof or even within the detector. In an arrangement where a sample ofthe atmosphere is conveyed to the detector through a duct, the emitter 2may be mounted in the duct or near the inlet thereof, so that theresulting aerosol will be conveyed to the detector.

I claim:

1. A system for detecting the presence, in an air atmosphere, ofparticulate combustion products and a selected gaseous contaminant, saidsystem comprising the combination of a first electrode and a secondperforate electrode defining a chamber within which said first electrodeis disposed and which is open to ambient atmosphere; a radioactivesource for dissociating air molecules within said chamber; circuit meansconnected across said electrodes for establishing current flowtherebetween when air molecules are dissociated within said chamber; andmeans for introducing to said chamber a vapor having the property ofchemcially reacting with said gaseous contaminant to form particles.

2. A system for detecting the presence in air of particulate combustionproducts and a gaseous contaminant selected from the group consisting ofS0 N0 HCl, HF, BrF and F said system comprising a particle detectorhaving an first electrode and a second electrode, said second electrodeforming a closed ionization chamber within which said first electrode isdisposed, a radioactive source within said closed ionization chamber, athird electrode having an opening therein and surrounding an extensionof said second electrode, said third electrode and said extensionforming an ionization chamber which is open to ambient air, aradioactive source within the open ionization chamber for dissociatingair molecules therein, and means for introducing to the interior of theopen ionization chamber a vapor having the property of reactingchemically with said gaseous contaminant to form particles.

3. The system of claim 2 wherein said means includes a vessel containinga sorbent for a volatile liquid precursor of said vapor, said sorbentincorporating a selected sorbed quantity of said precursor.

4. The system of claim 2 wherein said means includes a vessel containinga sublimable solid precursor of said vapor.

5. The system of claim 2 wherein said means includes a vessel whichcontains a charge of said vapor at a pressure exceeding the pressure ofsaid atmosphere.

6. The system of claim 2 wherein said means for introducing said vaporis carried by said particle detector.

7. The system of claim 2 wherein said means includes a vessel containinga volatile liquid precursor of said vapor.

8. The system of claim 7 wherein said means also includes a wick havingan end immersed in said liquid.

9. The system of claim 8 wherein said wick extends from within saidvessel into said sensing chamber.

10. The system of claim 8 wherein a portion of said wick is inliquid-transfer contact with a rigid porous member extending into saidsensing chamber.

1. A SYSTEM FOR DETECTING THE PRESENCE, IN AN AIR ATMOSPHERE, OFPARTICULATE COMBUSTION PRODUCTS AND A SELECTED GASEOUS CONTAMINANT, SAIDSYSTEM COMPRISNG THE COMBINATION OF A FIRST ELECTRODE AND A SECONDPERFORATE ELECTRODE DEFINING A CHAMBER WITHIN WHICH SAID FIRST ELECTRODEIS DISPOSED AND WHICH IS OPEN TO AMBIENT ATMOSPHERE, A RADIOACTIVESOURCE FOR DISSOCIATING AIR MOLECULES WITHIN SAID CHAMBER, CIRCUIT MEANSCONNECTED ACROSS SAID ELECTRODES FOR ESTABLISHING CURRENT FLOWTHEREBETWEEN WHEN AIR MOLECULES ARE DISSOCIATED WITHIN SAID CHAMBER, ANDMEANS FOR INTRODUCING TO SAID CHAMBER A VAPOR HAVING THE PROPERTY OFCHEMICALLY REACTING WITH SAID GASEOUS CONTAMINANT TO FORM PARTICLES. 2.A system for detecting the presence in air of particulate combustionproducts and a gaseous contaminant selected from the group consisting ofSO2, NO2, HCl, HF, BrF3, and F2, said system comprising a particledetector having an first electrode and a second electrode, said secondelectrode forming a closed ionization chamber within which said firstelectrode is disposed, a radioactive source within said closedionization chamber, a third electrode having an opening therein andsurrounding an extension of said second electrode, said third electrodeand said extension forming an ionization chamber which is open toambient air, a radioactive source within the open ionization chamber fordissociating air molecules therein, and means for introducing to theinterior of the open ionization chamber a vapor having the property ofreacting chemically with said gaseous contaminant to form particLes. 3.The system of claim 2 wherein said means includes a vessel containing asorbent for a volatile liquid precursor of said vapor, said sorbentincorporating a selected sorbed quantity of said precursor.
 4. Thesystem of claim 2 wherein said means includes a vessel containing asublimable solid precursor of said vapor.
 5. The system of claim 2wherein said means includes a vessel which contains a charge of saidvapor at a pressure exceeding the pressure of said atmosphere.
 6. Thesystem of claim 2 wherein said means for introducing said vapor iscarried by said particle detector.
 7. The system of claim 2 wherein saidmeans includes a vessel containing a volatile liquid precursor of saidvapor.
 8. The system of claim 7 wherein said means also includes a wickhaving an end immersed in said liquid.
 9. The system of claim 8 whereinsaid wick extends from within said vessel into said sensing chamber. 10.The system of claim 8 wherein a portion of said wick is inliquid-transfer contact with a rigid porous member extending into saidsensing chamber.